The present invention relates to an electronic circuit including a plurality of D/A converters and a liquid crystal display apparatus including the electronic circuit. Particularly, the present invention relates to an electronic circuit including a plurality of D/A converters each provided with a means for storing digital data for correcting offset value and an operation means for adding or subtracting the digital data for offset value correction to or from a digital input signal, and also a liquid crystal display apparatus including the electronic circuit.
As a conventional electronic circuit including a plurality of D/A converters, there is one as shown in FIG. 7 composed of only data latch circuits and D/A converters data. FIG. 8 is a block diagram of an example of liquid crystal display apparatus including such an electronic circuit of which plural D/A converters dac are used as means for transferring picture signals to a plurality of vertical data lines.
The above-mentioned conventional circuit has involved drawbacks as follows. More specifically, respective D/A converters have substantial fluctuations of performances, particularly remarkable differences in DC offset values (i.e., DC deviations from objective outputs) from respective amplifiers especially in case of using CMOS amplifiers at output stage analog buffer circuits. A certain vertical data line is always supplied with a signal from one D/A converter, so that the fluctuations of performances of D/A converters are recognized directly as fluctuations of picture display, more specifically as vertical band-like patterns corresponding to respective D/A converters, thus lowering the display quality.
On the other hand, a conventional type of liquid crystal apparatus includes a liquid crystal device comprising an active matrix substrate having thereon a plurality of data liens arranged in columns, a plurality of scanning lines arranged in rows, pixel electrodes each formed at an intersection of the data lines and the scanning lines, and pixel switches each formed at one pixel electrode so as to supply a picture signal to the pixel electrode from an associated data line via the pixel switch, a counter substrate disposed opposite to the active matrix substrate; and a liquid crystal disposed between the active matrix substrate and the counter substrate.
FIG. 18 is an equivalent circuit diagram for an active matrix-type liquid crystal device included in an active matrix-type liquid crystal display apparatus as an example of such a conventional liquid crystal apparatus. Referring to FIG. 18, the liquid crystal display apparatus includes a common signal line 701, vertical signal lines (data lines) 702-705, CMOS-type transfer switches 706-709, pixel switches 710, retention capacitors 711, a liquid crystal 712, a horizontal scanning circuit 713, a vertical scanning circuit 714, and scanning lines 715-718.
In such a liquid crystal display apparatus, it is generally practiced to apply AC signals for driving the liquid crystal so as to prevent the deterioration of the liquid crystal. FIG. 19 is a time-serial waveform chart showing a picture signal for such an AC drive wherein the picture signal is inverted for each cycle period to output both positive and negative polarity signals.
In such AC drive, a picture signal is caused to have a large amplitude on the order of, e.g., 9 volts, which is twice that of a positive or negative one-polarity signal relative to a central voltage. In FIG. 19, a portion of picture signal having a voltage higher than the central voltage is referred to as a positive-polarity signal, and a portion of the picture signal having a voltage lower than the central voltage is referred to as a negative-polarity voltage.
In such a liquid crystal display apparatus, such picture signals are supplied to the common signal line 701, and transfer switches 706-709 are sequentially turned on by the horizontal scanning circuit 713 to transfer the picture signals to the vertical signal lines 702-705 while one of the scanning lines 715-718 is sequentially selected by the vertical scanning circuit 714 to turn on the pixel switches 710 along a row of selected scanning line, thereby supplying the picture signals to the respective pixel electrodes on the row.
Heretofore, CMOS-type switches have been used as the transfer switches 706-709. On the other hand, if an n-channel-type MOS (transistor) switch is used as the transfer switch, the switch-on resistance is increased as the picture signal voltage becomes higher due to a substrate bias effect, so that sufficient signal transfer becomes difficult. Reversely, if a p-channel-type MOS switch is used, the switch-on resistance is increased as the picture signal voltage becomes lower due to the substrate bias effect, so that sufficient signal transfer becomes difficult.
For the above reason, a CMOS-type switch including both an n-channel and a p-channel has been used so as to attain a substantially constant on-resistance over an entire voltage range of picture signal.
On the other hand, regarding the AC drive inversion period, a row inversion, a column inversion or a dot inversion scheme, has been generally adopted. For each inversion in such cases, the polarity of a picture signal is inverted for each desired inversion period with respect to a central voltage as shown in FIG. 19 by a picture signal processing means, and then the picture signal is supplied to the common signal line 701.
Further, accompanying the demand for higher resolution pictures in recent years, the dot rate of picture signal is becoming very fast. Accordingly, a liquid crystal display apparatus as shown in FIG. 20 having two common signal lines 701A and 701B so as to reduce the dot rate to a half, has been also conventionally used. The number of input terminals can be further increased depending on a limiting operation frequency of the drive circuit.
However, even in such a liquid crystal display apparatus using two common signal lines for AC drive, a very high drive voltage on the order of 9 volts is required, thus requiring an increased power consumption. Further, as a result of using CMOS-type switches for the transfer switches, the circuit size has to be enlarged.
In view of the above-mentioned problems of the prior art, an object of the present invention is to provide an electronic circuit including a plurality of D/A converters and capable of uniformizing the performances of the D/A converters even when there are fluctuations in offset values for the respective D/A converters, by correction of the offset values.
A further object of the present invention is to provide a liquid crystal display apparatus including such an electronic circuit, thereby exhibiting a uniform display characteristic over the entire picture area.
A further object of the present invention is to provide a liquid crystal apparatus capable of operation at a relatively low drive voltage according to an AC drive mode.
According to the present invention, there is provided an electronic circuit, comprising a plurality of first D/A converters, wherein each first D/A converter is provided with:
an offset memory means for memorizing offset correction digital data, and
an operation means for adding or subtracting the offset correction digital data to or from a digital input signal to the D/a converter.
According to the present invention, there is further provided a liquid crystal apparatus, comprising:
a liquid crystal device comprising an active matrix substrate having thereon a plurality of signal lines arranged in columns, a plurality of scanning lines arranged in rows, and pixel electrodes each connected via a pixel switch to an intersection of the signal lines and the scanning lines so as to supply picture signals to the pixel electrodes via the signal lines, a counter substrate disposed opposite to the active matrix substrate, and a liquid crystal disposed between the active matrix substrate and the counter substrate, and
drive means for driving the liquid crystal devices, wherein said drive means including:
a first common signal line and a second common signal line for supplying the picture signals,
picture signal-supplying means for supplying picture signals of one polarity to the first common signal line and picture signals of the other polarity to the second common signal line,
a first and a second transfer switch provided to each column signal line for selectively supplying one of picture signals supplied to the first and second common signal lines to each column signal line, and
column inversion drive means for:
in a first frame, selectively turning on the first transfer switches for odd-numbered column signal lines and the second transfer switches for even-numbered column signal lines, and in a second frame, selectively turning on the second transfer switches for odd-numbered column signal lines and the first transfer switches for even-numbered column signal lines.
The present invention further provides a liquid crystal apparatus, comprising:
a liquid crystal device comprising an active matrix substrate having thereon a plurality of signal lines arranged in in columns, a plurality of scanning lines arranged rows, and pixel electrodes each connected via a pixel switch to an intersection of the signal lines and the scanning lines so as to supply picture signals to the pixel electrodes via the signal lines, a counter substrate disposed opposite to the active matrix substrate, and a liquid crystal disposed between the active matrix substrate and the counter substrate, and
drive means for driving the liquid crystal devices, wherein said drive means including:
a first common signal line and a second common signal line for supplying the picture signals,
picture signal-supplying means for supplying picture signals of one polarity to the first common signal line and picture signals of the other polarity to the second common signal line,
a first and a second transfer switch provided to each column signal line for selectively supplying one of picture signals supplied to the first and second common signal lines to each column signal line, and
dot inversion drive means for:
in a first frame, selectively turning on the first transfer switches for odd-numbered column signal lines and the second transfer switches for even-numbered column signal lines at the time of scanning odd-numbered scanning lines, and selectively turning on the second transfer switches for odd-numbered column signal lines and the first transfer switches for even-numbered column signal lines at the time of scanning even-numbered scanning lines; and
in a second frame, selectively turning on the second transfer switches for odd-numbered column signal lines and the first transfer switches for even-numbered column signal lines at the time of scanning odd-numbered scanning lines, and selectively turning on the first transfer switches for odd-numbered column signal lines and the second transfer switches for even-numbered column signal lines at the time of scanning even-numbered scanning lines.